The remains of the warship Vasa, which is around 400-years old – one of Sweden’s most popular tourist attractions with over 1 million visitors a year – could be significantly weaker than previously thought. But if you’re planning a trip to Stockholm in the near future, don’t worry: the ship isn’t in immediate danger.

The Vasa was built in 1628 during Sweden’s golden age for King Gustavus Adolphus and sank on its maiden voyage, just 2km from its dock. For the time, it was an enormous ship, capable of carrying 300 soldiers and firing a broadside of almost 600 pounds. But its construction seems to have been dogged by too many political considerations meaning it was built top-heavy. When the wind blew above a light breeze as it was manoeuvring in Stockholm harbour, that was it.

The Vasa was sprayed with polyethylene glycol to preserve the oak timbers. Credit: Ingela Bjurhager

For more than 300 years, the Vasa lay under the cool waters of the Baltic, until she was raised in 1961 to widespread media interest. Those cool waters are believed to have helped keep the oak timbers intact. Since then, the Vasa has been dried out, sprayed with polyethylene glycol (PEG) and housed in a museum.

PEG replaces water in the cells of the wood, strengthening it and stopping the shrinkage that would occur while it was drying. Now, a team of researchers from institutions in Stockholm and Uppsala have measured the tensile strength of portions of the wood taken from different parts of the ship. They found that the strength is roughly proportional to the molecular weight of PEG – the longer the polymer chains the better preserved that portion of the ship is likely to be. But the molecular weight of the polymer – and thus strength – is inversely proportional to the amount of iron present in the wood. The iron, in case you were wondering, comes from the iron nails and rivets that held the ship together.

So what’s going on between the iron and PEG? Ingela Bjurhager and colleagues suggest that, while the ship was drying out, iron and oxygen reacted with cellulose and lignin in the wood to produce an acid. The acid, they think, is responsible for the degradation of the polymer and the weakening of the wood – by up to 80% in some areas. Although the Vasa isn’t about to fall apart, the researchers do caution that the ‘risk of failure cannot be disregarded’.

UK readers may be thinking about our own salvaged historical wooden museum ship, the Mary Rose. She sank during the Battle of the Solent in 1545 after over 30 years’ service as Henry VIII’s favourite, only to rise again in 1982 – another televised salvage (which I remember watching as a child!). PEG was also used in treating the timbers, but from what Wikipedia tells me, they were not allowed to dry out beforehand, so it may retain such strength as it now has for longer. Neatly, I see that the Mary Rose will be unveiled in a new museum later this year – I’ll report back after a visit! (Full disclosure: my father-in-law is involved in the Mary Rose Trust.) Hopefully, my adult self will be more impressed seeing it than I was during a previous visit as a child seeing the hull being showered with water or PEG…

Neil Withers

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